It is proposed to use standard protein X-ray crystallographic techniques to determine the structure of human serum transferrin. This glycoprotein, which has a molecular weight of 80,000 daltons, has been crystallized in this laboratory, and work toward the low-resolution structure is in progress. The major objective of this proposal is to complete the structural analysis at 6 A resolution and to initiate work toward the high-resolution (2.8 A units) crystal structure. The low-resolution analysis should show the shape of the molecule and may reveal the relative positions of the two iron-binding sites. The high-resolution analysis, which will be aided considerably by current investigations of the primary sequence of human serum transferrin, will help answer the many questions that have been raised in the literature concerning the chemical and physical properties of this important iron-transport protein.